A Magnetless 4-Port Circulator and Its Microstrip Implementation

A magnetless 4-port circulator and its microstrip implementation have been presented in this brief. Compared to the reported magnetless 3-port circulators, the proposed 4-port circulator consists of four time-varying resonators whose resonances are spatio-temporally modulated with an incremental modulation phase of 90°. Analysis based on 4-port full-spectra coupling matrix shows directional RF signal transmission in this circuit, achieving a non-reciprocal transmission behavior. To validate the proposed concept, a 4-port distributed circulator is demonstrated by using the cross-type microstrip resonator with loaded time-varying capacitors at its four open-ends. The measurement results agree well with the numerical and simulation predictions. The circulator shows a non-reciprocal response with a transmission insertion loss of 3.4dB, a return loss of 15.6dB, and isolation of 27.6dB and 15.2dB at 1.0GHz.